Mariner 2

Mariner 2, the second in the Mariner series
of US unmanned interplanetary probes, was the first spacecraft to successfully
encounter another planet. It flew past Venus
at a distance of 35,000 kilometers (22,000 miles) on December 14, 1962,
sending back data on the temperature distribution of the planet's surface
and making basic measurements of Venus' atmosphere.

Mariner 2 was an exact copy of its sister craft, Mariner 1, which was lost
when its launch vehicle had to be destroyed. Both these early Mariners were
stripped down versions of the Ranger probes
used to investigate the Moon.

launch date/time

August 27, 1962; 06:53:14 UTC

launch vehicle

Atlas-Agena

launch site

Cape Canaveral Air Force Station

size

1.04m across hexagonal base
3.66m high

mass

203 kg

The flight of Mariner 2

Mariner 2 was launched by a two-stage Atlas-Agena rocket. After separation
of the Atlas first stage, the Agena booster fired for the first time to
put Agena-Mariner 2 into a 118km-high Earth parking orbit. A second burn
by Agena, 980 seconds later, followed by separation of the booster, put
Mariner 2 onto an Earth escape trajectory. Three-quarters of an hour later,
Mariner's solar panels opened out, to supply the spacecraft with power it
would need to run its science experiments, which began to be turned on a
couple of days later.

On Sep. 8, Mariner suddenly lost attitude control, possibly due to collision
with a small object, but this was restored by the onboard gyroscopes 3 minutes
later. En route to Venus, one of the solar panels failed but by this time
the spacecraft was close enough to the Sun to get adequate power from its
remaining panel. On Dec. 14, the probe's radiometers were turned on. Mariner
2 approached Venus from 30° above the dark side of the planet, and passed
below the planet at its closest distance of 34,773km at 19:59:28 UT Dec.
14, 1962. Its last transmission was received on Jan. 3, 1963. The probe,
now lifeless, remains in orbit around the Sun.

Scientific
harvest

Mariner 2 discovered the slow retrograde (backward) rotation rate for Venus,
hot surface temperatures and high surface pressures, a predominantly carbon
dioxide atmosphere, continuous cloud cover with a top altitude of about
60km, and no detectable magnetic field. It also showed that in interplanetary
space the solar wind streams continuously and the cosmic dust density is
much lower than in the near-Earth region. Improved estimates of Venus' mass
and the value of the astronomical unit were made.

The
spacecraft and its instruments

Mariner 2 consisted of a hexagonal base, 1.04 meters across and 0.36 meters
thick, which housed the electronics for the science experiments, communications,
data encoding, computing, timing, and attitude control, and the power control,
battery, and battery charger, as well as the attitude control gas bottles
and the rocket engine. Atop the base was a tall pyramidal mast on which
the science experiments were mounted. Attached to either side of the base
were rectangular solar panel wings with a total span of 5.05 meters and
width of 0.76 meters. Attached by an arm to one side of the base and extending
below the spacecraft was a large directional dish antenna.

Mariner 2 was powered by two solar panels, one 183 cm by 76 cm and the other
152 cm by 76 cm. These powered the craft directly or recharged a 1,000 W-hr
sealed silver-zinc cell battery, which was used before the panels were deployed,
when the panels were not illuminated by the Sun, and when loads were heavy.
Communications consisted of a 3W transmitter capable of continuous telemetry
operation, a high-gain directional dish antenna, a cylindrical omnidirectional
antenna at the top of the instrument mast, and two command antennas, one
on the end of either solar panel, which received instructions for midcourse
maneuvers and other functions.

Propulsion for midcourse maneuvers was supplied by a monopropellant (anhydrous
hydrazine) 225N retro-rocket. Attitude control with a 1-degree pointing
error was maintained by a system of nitrogen gas jets. Overall timing and
control was performed by a digital Central Computer and Sequencer. Thermal
control was achieved through the use of passive reflecting and absorbing
surfaces, thermal shields, and movable louvers.

The scientific experiments were mounted on the instrument mast and base.
A magnetometer was attached to the top of the mast below the omnidirectional
antenna. Particle detectors were mounted halfway up the mast, along with
the cosmic ray detector. A cosmic dust detector and solar plasma spectrometer
detector were attached to the top edges of the spacecraft base. A microwave
radiometer and an infrared radiometer and the radiometer reference horns
were rigidly mounted to a 48 cm diameter parabolic radiometer antenna mounted
near the bottom of the mast. All instruments were operated throughout the
cruise and encounter modes except the radiometers, which were only used
in the immediate vicinity of Venus.